The present invention relates to toners, to be used for electrophotograph, each in which a coloring agent selected from a group consisting of at least yellow, magenta, cyan, and black is internally added, a production method thereof, and a full-color image forming apparatus using the same.
There have been known various image forming apparatuses as full-color image forming apparatuses.
As an example, a first color image forming apparatus comprises a latent image carrier and a plurality of developing devices which are arranged around the latent image carrier and each of which holds a toner in which a coloring agent selected from a group consisting of at least yellow, magenta, cyan, and black is internally added. In the first color image forming apparatus, an electrostatic latent image is formed on the image carrier and is developed with the toners by the developing devices so as to sequentially superpose unicolors, thereby forming a full color toner image. Then, the full color toner image formed by superposing unicolors is directly transferred to a recording medium at once and the full color toner image on the recording medium is fixed with heat and pressure. Alternatively, the full color toner image formed on the latent image carrier by color superposition is transferred to an intermediate transfer medium at once and, after that, is further transferred to a recording medium and the full color toner image on the recording medium is fixed with heat and pressure.
As another example, a second color image forming apparatus comprises a latent image carrier and a plurality of developing devices which are arranged around the latent image carrier and each of which holds a different unicolor toner. In the second color image forming apparatus, electrostatic latent images are formed on the image carrier and are sequentially developed with the respective toners by the developing devices. The toner images for the respective colors formed on the latent image carrier are sequentially transferred onto an intermediate transfer medium to superpose colors, thereby forming a full color toner image on the intermediate transfer medium. Then, the full color toner image is transferred to a recording medium at once and the full color toner image on the recording medium is fixed with heat and pressure.
As another example, a third color image forming apparatus comprises toner image forming means each of which is provided for each toner containing a coloring agent selected from a group consisting of at least yellow, magenta, cyan, and black. Each toner image forming means comprises a latent image carrier and a developing device wherein an electrostatic latent image is formed on the latent image carrier and is developed by the developing device. In the third color image forming apparatus, toner images of the respective colors on the latent image carriers are sequentially transferred to an intermediate transfer medium to superpose colors, thereby forming a full color toner image on the intermediate transfer medium. Then, the full color toner image is transferred to a recording medium at once and the full color toner image on the recording medium is fixed with heat and pressure.
In the aforementioned first color image forming apparatus, different unicolor toner images are superposed on the single photoreceptor. In the second color image forming apparatus, respective unicolor toner images formed on the single photoreceptor are superposed on the recording medium or the intermediate transfer medium so as to form a full color toner image. In the third color image forming apparatus, different unicolor toner images are formed on the photoreceptors, respectively and then superposed on the recording medium or the intermediate transfer medium so as to form a full color toner image.
Any of such color image forming apparatuses as mentioned above has such a problem that the transfer efficiency of toner images becomes insufficient during the superposition of unicolors. The insufficient transfer efficiency may cause the toner scattering and/or color irregularity, thus developing a color different from a desired color.
There is also a problem that, in case of applying a transfer voltage by a constant voltage power source to transfer formed toner images, not all the toner images are transferred correctly. For this, it is required to apply so-large transfer voltage.
As the amount of toners not used in image formation increases, the consumption of toners increases. In case that residual toners not transferred to the photoreceptor or the intermediate transfer medium are collected as waste toners by a cleaning device, the increase of residual toners not transferred leads to the increase of waste toners, thus accelerating the deterioration of cleaning members.
For storing a large amount of waste toners, a large-capacity waste toner container is required. The large-capacity waste toner container increases the volume of the image forming apparatus, making the fulfillment of requirement of reducing the size of the image forming apparatus impossible. This is also a problem.
In case of collecting untransferred toners in the developing device and reusing the toners in next development, as the amount of untransferred toners increases, the percentage of toner of which charging property and the like are deteriorated is increased, thus leading to affect on the characteristics of formed image. This is also a problem.
For heightening the resolution of formed color image and reducing the consumption of toners, toners having small particle diameters are used. However, diminish in particle diameter of toner lowers the fluidity of the toner. Particularly in case of non-magnetic single-component development, this makes triboelectric charge with the surface of a development roller or a regulating blade difficult, causing a problem of not imparting enough charge. Therefore, charge distribution is generated in toner so that it is inevitable that a negatively charged toner contains positively charged toner particles, thus causing a problem of fog in non-image portions of the image carrier. To prevent the occurrence of fog, it is known to increase the regulation pressure in case of non-magnetic single-component development. However, increase in the regulation pressure may cause excessive charge of toner, thus generating a tendency toward reduction in toner concentration during development and a tendency toward reduction in transfer efficiency.
For example in JP-A-06194943, in order to solve the aforementioned many problems, it has been proposed to limit the amount of adhering toner on the development roller after regulation in a proper range. Also, in JP-A-2002131973, it has been proposed to use small-particle toner and to define the upper limits in amount of the respective unicolor toners adhering to a recording medium in order to improve the charging property, the image quality, and the granularity. In JP-A-08248779, JP-A-2000206755, JP-A-200231933, JP-A-200231933, JP-A-05307310, and the like, for a full-color image, it has been proposed to determine the transfer sequence among unicolor toners of yellow, magenta, and cyan and black toners.
In JP-A-10207164, it has been proposed to start development with toner particles having small charge. In JP-A-10260563, it has been proposed to increase the transfer voltage for every color to improve the transfer efficiency. In JP-A-0527548, it has been proposed to set the transfer voltage so as to increase the transfer efficiency of toner particles in the lowest layer.
In JP-A-2000128534, it has been proposed to use hydrophobic rutile/anatase type titanium oxide to improve external additive particles, thus preventing external additives from being embedded due to friction. In JP-A-200183732, it has been proposed to set the percentage of rutile/anatase mixed crystal type titanium oxide and hydrophobic silica particles which strongly adhere toner mother particles to 90-98%, thereby achieving good triboelectric characteristic and thus obtaining a color image with no soilure due to toner scattering and no fog.
In JP-A-200122118, it has been proposed to add silica particles and titanium oxide particles as external additives to toner mother particles and to set the liberation ratio of silica to 0.5-8% and set the liberation ratio of titanium oxide to 0.5-5%, thereby preventing defects of transferred colorant in solid image, fog, and filming. In JP-A-200272544, it has been proposed to add silica particles and titanium oxide particles to toner particles with high degree of circularity, and to set the number liberation ratio of titanium oxide to 1.00-50.00% and set the number liberation ratio of silica to 0.01-4.00% such that the number liberation ratio of titanium oxide is larger than the number liberation ratio of silica.
In JP-A-08272132, JP-A-08314280, it has been proposed to add metallic soap (zinc stearate) as external additive so as to make developer which can exhibit excellent transfer efficiency, does not allow the generation of internal void phenomenon, and hardly produce fog. In JP-A-200210799, it has been proposed that the addition of metallic soap particles to toner is effective in extending the life of a photoreceptor.
Further in JP-A-11167224, it has been proposed to apply metallic soap not only to toner but also to a photoreceptor, thereby preventing the adhesion of toner account for scumming.
Furthermore in JP-A-08272228, it has been proposed to apply metallic soap to an intermediate transfer medium, thereby improving its separation characteristic relative to toner and thus improving the transfer efficiency.
In JP-A-11323396, it has been proposed to set the particle diameter of metallic soap externally added to toner mother particles to 4 μm or less, thereby improving the cleaning characteristic of toner.
Moreover in JP-A-200151443, it has been proposed to set the particle diameter of metallic soap to 5 μm or less and also use titanium oxide and silica particles with the metallic soap, thereby preventing the production of spent toner, the occurrence of filming, and the generation of blemishes in a photoreceptor.
In JP-A-2002169330, it has been proposed to externally add fatty calcium salt to toner mother particles produced in the polymerization method, thereby preventing the wear of a cleaning blade and thus preventing the passing of toner particles and the sticking of toner particles.
In JP-A-0611898, it has been proposed that in unicolor toners for forming a full color image, setting the difference in work function of two of the unicolor toners to be 0.5 eV or less enables the formation of an image which is excellent in color reproducibility.
Since there is a limit to improve the toner's transfer efficiency in a developer of which toner mother particles are treated with external additives, however, any of the aforementioned propositions can not extremely reduce the amount of waste toners and therefore requires a waste toner container of a certain level of size.
As another type of color image forming apparatus, there has been known an image forming apparatus of a tandem system without an intermediate transfer medium so that a toner image is directly transferred to a recording medium. The tandem system allows the reduction in size of the image forming apparatus and also enables the high-speed formation of a color image.
In the tandem-type image forming apparatuses, the transfer sequence or the like has been considered in order to form a color image having excellent image quality. For example, in JP-A-09319179, it has been proposed to adjust the amount of adhering toner on a photoreceptor to be previously transferred to be larger than the amount of adhering toner on the photoreceptor to be transferred later in the process of transferring an image to a recording medium, thereby providing a color image having excellent color balance.
This is because adjusting the amount of adhering toner to be previously developed and transferred to be larger than that of the next one prevents the disruption of color balance due to reduction in amount of previously transferred toner because some toner particles are reversely transferred to the photoreceptor for the next color during the development with the next color. However, this proposition does not prevent the generation of reversely transferred toner particles as a cause. That is, this proposition does not prevent the generation of reversely charged toner particles.
In JP-A-0764366, it has been proposed to increase the adhesion force between toner and a transfer medium by pressing the toner on the transfer medium after transfer with pressure means from both sides of the transfer medium, thereby preventing the generation of reversely transferred toner particles.
Since it is required to separately provide at least three pressure means, however, the size of the image forming apparatus must be increased, making the fulfillment of requirement of reducing the size of the image forming apparatus impossible.
In JP-A-2000242152, it has been proposed to provide a reversely transferred toner removing means having a polarity opposite to that of toner at a downstream side of a transferred portion of each of photoreceptors at least from the second one to the last one from the upstream side of the traveling direction of a feeding body for recording media in order to remove the reversely transferred toner particles.
However, this method can not prevent the generation of reversely transferred toner particles so that the amount of waste toner is increased and a large-capacity waste toner container is therefore required. The large-capacity waste toner container does not allow the reduction in size of the image forming apparatus.
In order to improve the image quality of formed color images, unicolor toners having relatively small particle diameter are used. In order to improve the transfer efficiency, conglobated toner is used. Since diminish in particle diameter of toner lowers the fluidity of the toner, however, this makes triboelectric charge with the surface of a development roller or a regulating blade difficult, causing a problem of not obtaining enough charge. Therefore, uneven charge distribution is generated in toner so that it is inevitable that even a negatively charged toner contains positively charged toner particles, thus causing a problem of fog in non-image portions of the image carrier.
In order to prevent the occurrence of fog, it is known to increase the regulation pressure in case of non-magnetic single-component development. However, increase in the regulation pressure may cause excessive charge of toner, thus generating a tendency toward reduction in toner concentration during development and a tendency toward reduction in transfer efficiency.
In JP-A-06194943, JP-A-08297413, JP-A-09062030, JP-A-11218957, and the like, it has been proposed to limit the amount of adhering toner on a development roller after regulation in a proper range. However, it is difficult to prevent the occurrence of fog and the generation of reversely transferred toner.
In JP-A-2002131973, a full color image forming method has been proposed to use small-particle toner and to define the upper limits in amount of the respective unicolor toners adhering to a recording medium in order to improve the charging property, the image quality, and the granularity. Though this method is effective in improvement of low-temperature fixing property for uniformly fusing toner, it is not effective in prevention of generation of reversely transferred toner.
In JP-A-07306564, an image forming apparatus has been proposed which is an image forming apparatus in which images are transferred from a plurality of image carrier drums sequentially arranged to a recording medium and is characterized in that an image carrier drum for forming a black toner image is arranged at the most upstream side of a feeding belt for feeding a recording medium.
Also in JP-A-7306564, a color image forming apparatus has been proposed in which the first development is conducted with yellow toner at the most upstream side and the final development is conducted with black toner at the most downstream side.
These are for preventing problems such as retortion of a cleaning blade only during the development with black toner and are not effective in prevention of the occurrence of fog and the generation of reversely transferred toner particles and in prevention of color registration error.
In JP-A-2002258567, JP-A-08227171, JP-A-2000003068, and the like, it has been proposed that a toner which is excellent in transfer property and cleaning property and hardly causes the scattering of toner particles is obtained by combination of toner particles of which average degree of circularity is high, silicide particles and silica particles which have different particle diameters.
However, none of these has been made for preventing the generation of reversely transferred toner particles.
It is an object of the present invention to provide a plurality of unicolor toners with which the color superposition is conducted during the development on a latent image carrier on which an electric latent image is formed or the color superposition is conducted during transfer to a recording medium after development, a production method thereof, and an image forming apparatus employing the toners, wherein the toners can provide high transfer efficiency, can prevent the scattering of toner particles, color registration error and toner dispersal, disturbance in transferred image, defects of transferred colorant, and uneven transfer, can exhibit excellent color reproducibility, enables the extreme reduction in amount of waste toner collected by cleaning, enables the reduction in apparatus size, and can extend the lives of a latent image carrier and a cleaning blade so as to achieve the low running cost.
It is another object of the present invention to provide an image forming apparatus of tandem system in which toner images formed on latent image carriers are sequentially transferred to a recording medium fed by a feeding belt, wherein the transfer of a toner image is not affected by a toner image which was previously transferred and the image forming apparatus does not allow the occurrence of color registration error among the respective unicolor toner images, can provide excellent color reproducibility, and does not allow the occurrence of scattering of toner particles and the generation of reversely transferred toner particles.